Captive Portal Pre-authentication Redirect

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The Pre-authentication redirect URL setting in a Captive Portal zone redirects the user to a landing page hosted on a different server (local or remote). The user is then sent back to the portal login URL for authentication by clicking a link or similar process on the landing page.

This is often used by businesses to ensure a site-specific page is presented to users before anything else, including the login page.

To use the Pre-authentication redirect URL, the Captive Portal HTML/PHP page must handle the redirect to the destination page, and the IP address(es) of the target server must be added as Captive Portal Allowed IP Addresses or Allowed Hostnames if the target server is on a separate subnet. It isn't as easy as doing a post-authentication redirect to another page.

The portal page must contain code at the top of the page similar to the following:

<?php
require("globals.inc");
$request_uri = urldecode(str_replace("/index.php?redirurl=", "",  $_SERVER["REQUEST_URI"]));
$portal_redirurl = urldecode("$PORTAL_REDIRURL$");
if(!stristr(urldecode("$PORTAL_REDIRURL$"), $request_uri)) {
	Header("Location: $PORTAL_REDIRURL$");
	exit;
}
?>
[Rest of CP login page]

The user will be redirected to that URL, and then when the user clicks back to pfSense using a link to the firewall IP address/port, such as http://x.x.x.x:8000/index.php, the referring URL will match the original landing page and the redirect will not happen that time. The above code will not work for every case, it may need adapted to suit site-specific needs.

Adapted from https://forum.pfsense.org/index.php?topic=34148.msg181641#msg181641